The R-T-B based sintered magnet (R represents a rare earth element, T represents one or more elements of iron group elements containing Fe as an essential, and B represents boron), a representative of which is Nd—Fe—B based sintered magnet, is advantageous for miniaturization and high efficiency of the machines using it due to high saturation flux density, and thus can be used in the voice coil motor of the hard disk drive and the like. In recent years, the R-T-B based sintered magnet also has been applicable in various industrial motors, or driving motors of hybrid vehicles, or the like. From the viewpoint of energy conservation and the like, it is desirable that the R-T-B based sintered magnet can be further popularized in these fields. However, when applied in the hybrid vehicles and the like, the R-T-B based sintered magnet will be exposed to a relatively high temperature. Therefore, inhibition of the high temperature demagnetization caused by heat becomes important. For inhibition of the demagnetization under high temperature, a method for sufficiently improving coercivity of the R-T-B based sintered magnet at room temperature is well known as effective. In addition, the iron group elements in the present specification mean Fe, Co and Ni.
For example, as a method for improving a coercivity of the Nd—Fe—B based sintered magnet at room temperature, a method in which part of Nd of the compound Nd2Fe14B as the main phase is replaced with heavy rare earth elements such as Dy, Tb and the like is well known. By replacing part of Nd with the heavy rare earth elements, the magnetic anisotropy of crystals is increased, and as a result, the coercivity of the Nd—Fe—B based sintered magnet at room temperature can be sufficiently improved. In addition to the replacement with heavy rare earth elements, addition of elements such as Cu and the like is also effective in improving coercivity at room temperature (Patent Document 1). By adding the element Cu, the phase such as Nd—Cu liquid phase can be formed by the element Cu at the grain boundary, and thus it is considered that the grain boundary become smooth and formation of reverse magnetic domains is inhibited.
However, in the R-T-B based rare earth based magnet, the ideal existing form of the R2T14B as the main phase was pointed out at the initial stage of development. In Patent Document 2, it is described that ‘as the existing form of the tetragonal compound, it is ideal that the fine particles having high anisotropy constant are isolated by the non-magnetic phase’.